In recent years, the Thin Film Transistor-LCD (TFT-LCD) has been rapidly developed and applied widely. For the TFT-LCD in the mainstream market, three types, which respectively are Twisted Nematic (TN), Super Twisted Nematic (STN), In-Plane Switching (IPS) and Vertical Alignment (VA) can be illustrated. The VA liquid crystal display possesses extremely high contrast than the liquid crystal displays of other types, which can reach up to 4000-8000 in general. It has very wide application in large scale display, such as television or etc.
The reason why the VA liquid crystal display possesses extremely high contrast is that the liquid crystal molecules are vertically aligned to the substrate surface, and no phase difference exists, and light leakage is very small, and the dark state brightness is extremely small at the dark state without applying electricity. The lower the brightness at the dark state can be, the higher the contrast is according to the contrast calculation formula. For vertically aligning the liquid crystal molecules of the VA liquid crystal display to the substrate surface, it is demanded to implement vertical alignment treatment to the liquid crystal molecules. The most common way is to coat vertical alignment solution on specific areas of surfaces of the upper, lower substrates, and the alignment solution generally comprises a large amount of chemical solution NMP (N-methylpyrrolidone) and Polyimide (PI), and then to bake the substrates for a long period at high temperature (generally above 200 degrees) for curing the solvent in the alignment solution. Thus, PI alignment layers are formed on the surfaces of the substrates. As shown in FIG. 1, the traditional VA type liquid crystal display panel comprises: an upper substrate 10, a lower substrate 20 oppositely positioned to the upper substrate 10, a liquid crystal layer 50 sandwiched between the upper substrate 10 and the lower substrate 20 and PI alignment layers 30 formed at one side of the upper substrate 10 facing the lower substrate 20 and one side of the lower substrate 20 facing the upper substrate 10. However, because the VA liquid crystal display utilizes vertical twist liquid crystals and the birefraction difference of the liquid crystal molecules is larger, the issue of the color shift under large view angle is more serious.
For earning better wide view angle property for the VA liquid crystal display panel to improve the color shift issue, the multi-domain VA (MVA) technology is commonly utilized, which is to divide a sub pixel into many districts and drive the liquid crystals in respective districts to lie down toward different directions as applying voltage. Thus, the watch results from respective directions can be equal. There are many methods for realizing the MVA technology. Please refer to FIG. 2, FIG. 3 and FIG. 4. One of the methods is to process onside of the ITO pixel electrode 70 to be a pozidriv pattern. The common electrode 80 is a plane electrode which has uniform thickness and is uninterruptedly continuous. With the special ITO pixel electrode pattern, the tilt electric field can induce the liquid crystal molecules 50 to fell down toward different directions.
FIG. 2 is a top view diagram of one side of a lower substrate 20 in an MVA liquid crystal display panel. 210 and 220 respectively are a scan line and a data line. One sub pixel is divided into four areas by the pixel electrode 70. The ITO pixel electrode 70 comprises a pozidriv keel 701 and a pattern of pixel electrode branches 702 respectively extending in directions of 45°, 135°, −45° and −135° from the pozidriv keel 701 relative to the horizontal direction with spaced slits. FIG. 3 is a sectional diagram of an MVA type liquid crystal display panel corresponding to A-A portion shown in FIG. 2. The pixel electrode 70 with slits is positioned on the flat lower passivation layer 60. The plane common electrode 80 is positioned on the flat upper passivation layer 90. The PI alignment layers 30 cover on the pixel electrode 70 and the common electrode 80.
According to the transmittance formula of the VA liquid crystal display panel:
                    T        =                              1            2                    ⁢                      sin            2                    ⁢          2          ⁢          ΔΦ          ⁢                                          ⁢                      sin            2                    ⁢                      Γ            2                                              (        1        )            
wherein T is the penetration rate, and AO is the included angle between the long axis of the liquid crystal and the polarizer, of which the efficiency is the maximum as the angle is 45°; Γ is the phase difference, i.e. the modulation result to the polarized light with the liquid crystal molecules driven by the electrical field.
the calculation formula of Γ is:Γ=cos(a)*2π*Δn*d/λ  (2)
wherein a is the included angle between the long axis of the liquid crystal and the normal line of the substrate, and the volume is determined according to the electrical fielding affecting the liquid crystal molecules, and d is the cell gap, and Δn is the refractivity difference of the long, short axes of the liquid crystal.
According to the penetration rate formula, in the four areas of the sub pixel, the pixel electrode 70 comprises a pattern of pixel electrode branches 702 respectively extending in directions of 45°, 135°, −45° and −135° relative to the horizontal direction with spaced slits (the direction of the upper, lower polarizers respectively are 0°, 90°). The long axes of the liquid crystal molecules will respectively fell down toward the directions of 45°, 135°, −45° and −135° relative to the horizontal direction. In the penetration rate formula sin2 2ΔΦ=1, the maximization of the penetration rate can be achieved.
However, the liquid crystal molecules 50 in the area corresponding to the pozidriv keel 701 of the pixel electrode 40 as shown in FIG. 2 always cannot fell down as that the liquid crystal molecules in the areas corresponding to the pattern of the pixel electrode branches 702 with spaced slits fell down toward the directions of 45°, 135°, −45° and −135° relative to the horizontal direction. Thus, as shown in FIG. 5, the liquid crystal molecules 50 in the area corresponding to the pozidriv keel 701 tilt toward 0° or fell down toward 90° to make sin2 2ΔΦ=0 in the penetration rate formula. The display is in an opaque state to cause the entire penetration rate of the liquid crystal display panel to descend.